StarDate Podcast

Early in its history, the inner solar system was chaotic. Violent collisions might have destroyed many small worlds, while perhaps creating others – including the Moon. It probably formed when a planet as big as Mars rammed into Earth, blasting out debris that came together to make the Moon. A recent study says that a meteorite discovered a few years ago might be a remnant of one of the demolished worlds. NWA 15915 was discovered in Algeria. Scientists analyzed the composition, structure, and magnetic properties of the six-pound meteorite. They concluded that it’s a rare type of meteorite – it doesn’t come from any known asteroid, planet, or moon. But it does have some similarities to Mercury, the smallest planet and the one closest to the Sun. The study suggests that NWA 15915 might have come from a Mercury-like planet born in the same region of the solar system. The planet was demolished long ago by a giant impact. But a few fragments remain. The findings are preliminary. So it’ll take more work to confirm that a piece of a dead planet fell atop the desert sands of northwestern Africa. Mercury itself is near the Moon this evening. It looks like a fairly bright star to the left of the Moon. They’re quite low in the sky as twilight fades, so you need a clear horizon to spot them. Script by Damond Benningfield

It would be fascinating to get close to Cygnus X-3. Unfortunately, it also would be deadly. The system is bathed in X-rays and ultraviolet radiation. It features powerful “jets” that blast into space like energy cannons. And it probably has a black hole – a one-way trip to oblivion. Cygnus X-3 is in the swan, which swoops across the eastern sky on these early summer evenings. The system itself is too faint to see. In fact, we can’t see its visible light even with the largest telescopes because it’s hidden behind thick clouds of dust. But we can see it in other wavelengths, including X-rays, gamma rays, and radio waves. Those forms of energy have allowed astronomers to piece together the system’s likely story. Cygnus X-3 probably consists of a black hole plus a brilliant companion. The companion probably is a dozen or more times the Sun’s mass, and a couple of hundred thousand times its brightness. The bright star is blowing huge amounts of gas into space. The black hole grabs some of the gas, which forms a spinning disk around the black hole. Some of the gas is funneled into high-speed jets that fire into space. One of those jets is aimed almost directly at Earth. The brilliant companion star is likely to explode in the next million years or so, with its core collapsing to form another black hole. But the blast might rip the system apart – perhaps causing Cygnus X-3 to fade away. Script by Damond Benningfield

425 years ago, a “new” star flared to life near the neck of Cygnus, the swan. The star slowly faded, then flared twice more during the 17th century. It’s remained visible ever since. And someday soon, it’ll flare up again – for the last time: It’ll explode as a supernova. P Cygni is more than 5,000 light-years away, so it must be extremely bright for us to see it at all. And in fact, it’s one of the brightest stars in the entire galaxy – 600 thousand times brighter than the Sun. P Cygni is so brilliant because it’s 35 to 40 times the mass of the Sun. Such a monster burns through the nuclear fuel in its core in a hurry. So even though P Cygni is only a few million years old – compared to four and a half billion years for the Sun – it’s nearing its end. The earlier outbursts might have erupted because the star’s interior is unstable. It gets so hot that the star blasts some of the gas at its surface into space. There’s evidence that similar outbursts took place thousands of years earlier. P Cygni is likely to explode within a couple of million years. Its core might collapse to form a super-dense neutron star – or even a black hole. Under dark skies, P Cygni is visible to the eye alone. At nightfall, it’s in the east-northeast, close to the right of Sadr, the bright star that connects the swan’s body to its wings. More about the swan tomorrow. Script by Damond Benningfield

Cygnus, the swan, soars gracefully through summer nights. Its brightest star, Deneb, is in the northeast at nightfall. It marks the swan’s tail. The swan’s body stretches to the right, parallel to the horizon. The wings extend above and below, connected to the body by the star Sadr. Cygnus contains many star clusters. The list includes several that stretch from Sadr to the south, roughly along the swan’s neck. The clusters contain a few dozen to a few hundred stars. All of them are young – no more than about 10 million years old. And many of them are especially hot, bright, and massive. The clusters are indirectly related. They belong to much larger collections of young stars, plus the raw materials for making more stars. A “wave” passed through that region of the galaxy, squeezing gigantic clouds of gas and dust. Clumps of material within the clouds collapsed, forming stars. Over the next few million years, the most massive stars will explode as supernovas. Shockwaves from the blasts may compress more pockets of gas and dust, creating more stars. But the clusters themselves won’t survive much longer – at least on the galactic timescale. They’ll be pulled apart by the gravity of the surrounding stars and clouds, so their stars will go their separate ways. The clusters are easy targets for good binoculars. One is just a whisker from Sadr. Several others trail off to the right – sparkly decorations for the swan. Script by Damond Benningfield

The Royal Observatory at Greenwich has been one of the most important skywatching sites in history – not so much for what it told us about the stars, but for its role right here on Earth. Its location marked the starting point for measuring longitude – the position east and west on the globe. It also marked the time standard for the entire world: Greenwich Mean Time. The observatory was established on today’s date in 1675, by King Charles II. It was built on a hill near London, overlooking the Thames. Greenwich was created to provide highly accurate maps of the stars, and to measure the motions of the Sun, Moon, and planets. The work was designed to help sailors determine their longitude. Establishing longitude at sea was extremely difficult – and dangerous; many ships crashed because their navigators didn’t know where they were. The observations also played a key scientific role: they helped confirm that the motions of the Sun, Moon, and planets were governed by Isaac Newton’s laws of gravity. In 1833, the observatory began a “time service.” It dropped a ball from a tall pole at precisely 1 p.m. That allowed mariners to set their clocks before they sailed. Greenwich later transmitted the time to the whole country by telegraph, then radio. The observatory was moved in the 20th century, and closed in 1998. Today, the Greenwich site is a museum – preserving an important part of world history. Script by Damond Benningfield

The only astronomical object most of us notice in the daytime sky is our star, the Sun. Its light makes the sky bright, which overpowers the other stars and almost everything else. But there are a couple of exceptions: the Moon and the planet Venus. During its 29-and-a-half-day cycle of phases, the Moon spends half of its time in the daytime sky – the hours between sunrise and sunset. But it’s so pale compared to the Sun that it can be hard to notice. That’s especially true when the Moon is in its crescent phase, as it is now. At dawn tomorrow, the Sun will light up only about one-eighth of the lunar hemisphere that faces our way. So the Sun will look millions of times brighter than the Moon. As twilight begins, the Moon will continue to dominate the sky. But as twilight gets brighter, the Moon will appear to fade. It’s still just as bright, but against the daytime sky it’s hard to see. But you can still spot it, leading the Sun across the sky. At dawn, Venus stands to the lower right of the Moon. It’s the brilliant “morning star.” Until sunrise, only the Moon outshines it. Venus also fades into the blue of the daytime sky. But it is visible to the unaided eye. It leads the Moon up the sky in the morning, and down the sky in the afternoon. It’s hard to find, but once you see it, you’ll wonder why you never noticed it before – a bright planet shining through the bright blue sky. Script by Damond Benningfield

The Sun is at a standstill. Oh, it’s still orbiting the center of the galaxy at an impressive clip – about half a million miles per hour. And it’s still moving across the sky as Earth turns on its axis. But the points along the horizon at which the Sun rises and sets aren’t changing. That’s because today is the summer solstice. It’s a point in Earth’s orbit that marks the beginning of summer in the northern hemisphere and winter in the southern hemisphere. We have seasons because Earth is tilted on its axis. At the June solstice, the north pole tilts toward the Sun, bringing more sunlight to the northern hemisphere. Six months later, the south pole tilts toward the Sun, giving the northern half of the globe shorter days and longer nights. Between the solstices, the Sun moves north and south in the sky. So its rising and setting points move north and south as well. At some times of year, if you have a good way to mark these points, you can see the difference from day to day. But the Sun appears to “stand still” along the horizon for a few days on either side of the solstice. In fact, the word solstice means “Sun stands still.” At the June solstice, the Sun is farthest north for the year, so it rises and sets to the north of due west. Just how far north depends on your latitude. Incidentally, the summer solstice is also the longest day of the year, so there’s plenty of sunlight as we head into summer. Script by Damond Benningfield

The number of confirmed planets in other star systems has reached about 6,000. But few of those planets are likely homes for life. Most are too hot, too cold, too “gassy,” or they’re zapped by too much radiation by their star. A few planets are in the “well, maybe” category. They might be suitable for life, but the conditions aren’t perfect. An example is a planet in the star system 82 Eridani. The system is about 20 light-years from Earth, and its star is similar to the Sun. Astronomers have confirmed three planets in the system, with hints of more. Two of the planets are quite close to the star, so they’re too hot for life like that on Earth. But the third planet is more intriguing. It’s about six times the mass of Earth, so it could be dense and rocky. Its average distance from the star is about a third farther than Earth’s distance from the Sun. At that range, the planet spends most of its time in the star’s habitable zone – the region where conditions are most comfortable for life. But the planet’s orbit is so lopsided that the distance varies by more than a hundred million miles. So as the planet moves around 82 Eridani, surface temperatures range from hot enough to boil water to cold enough to freeze the entire surface. That makes it unlikely that anything lives on the planet. It is possible that life could exist below the surface – avoiding the extremes on this “yo-yoing” planet. Script by Damond Benningfield

Astronomers have been searching for planets around one of our closest neighbor stars for decades. And they’ve reported the discovery of several. But the reports have come to naught – until now. Earlier this year, a team confirmed the presence of four planets – all of them smaller than Earth. Barnard’s Star is six light-years away. Only the three stars of the Alpha Centauri system are closer. The star is much smaller and less massive than the Sun, and less than one percent as bright. In fact, it’s so faint that it wasn’t discovered until a little more than a century ago. Barnard’s Star is ancient – probably twice the age of the Sun or older. So if it has planets, there’s been plenty of time for life to take hold. That’s made finding planets a high priority. Last year, a team of astronomers confirmed one planet, and said there might be three more. All of those were confirmed in March. None of the planets is more than a third the mass of Earth. And they’re so close in that they orbit the star in a week or less. So even though Barnard’s Star is faint, the planets are all too hot to provide comfortable conditions for life. Barnard’s Star is in Ophiuchus, the serpent-bearer. The constellation stretches across the east and southeast in early evening, and stands high in the south later on. But Barnard’s Star is too faint to see without a telescope. We’ll have more about exoplanets tomorrow. Script by Damond Benningfield

If you stare at one of the giant planets of the outer solar system long enough, with a big enough telescope, you’re likely to find some moons. That was certainly the case a couple of years ago for Saturn. A research team scanned the space near Saturn with a large telescope in Hawaii. And earlier this year, the team reported its results: a haul of 128 previously unseen moons. That brought the planet’s total to 274. That’s three times the number of moons for second-place Jupiter – at least for now. The newly found moons are small and faint – no more than a few miles in diameter. They follow odd orbits, including some that orbit backwards – in the opposite direction from Saturn’s rotation. Some of the moons may be chunks of space rock that were captured by the giant planet’s gravity. Others may be the remains of larger moons that were blasted apart by collisions. About a third of the moons may be the remnants of a single impact. They form a group named Mundilfari for a Norse god related to Saturn. It’s possible the impact took place within the past hundred million years – adding lots of little moons to Saturn’s family. Look for Saturn near our own moon the next couple of mornings. The planet looks like a bright star. It’ll stand to the lower left of the Moon at dawn tomorrow, and closer to the right of the Moon on Thursday. Tomorrow: a passel of planets for a nearby star. Script by Damond Benningfield

If you sit on a big beachball, it gets mashed down. That makes it a little wider through the middle, and a little narrower from top to bottom. And that makes it look a lot like Alderamin, the brightest star of the constellation Cepheus the king. The star is about a third wider through the equator than through the poles. That’s not because some cosmic giant is sitting on it. Instead, it’s because the star spins like crazy. Alderamin is about 50 light-years away, so it’s a fairly close neighbor. It’s nearing the end of the prime phase of life, even though it’s billions of years younger than the Sun. That’s because it’s twice as massive as the Sun. Heavier stars “burn” through their nuclear fuel much faster than lighter stars. What really stands out about Alderamin, though, is its shape. The star’s equator rotates once every 12 hours, versus almost four weeks for the Sun. That forces gas outward around its middle, making the star look a bit more like a fat lozenge than a ball. As Alderamin continues to age, though, it will puff up to many times its current diameter. That will slow down its high-speed rotation, giving Alderamin a “rounder” appearance. Cepheus is in the north and northeast at nightfall. Under fairly dark skies it’s easy to make out. It looks like a child’s drawing of a house. The peak of the roof is on the left during the evening, with Alderamin marking the top right corner of the sideways house. Script by Damond Benningfield

Capricornus may be the most inventive constellation of the zodiac. For one thing, all of its stars are faint, so it takes some work to see any kind of pattern there. And for another, it represents the oddest creature in the heavens: a sea-goat – the front half of a goat plus the tail of a fish. It’s easy to find the sea-goat’s location early tomorrow, because the Moon passes quite close to its brightest star. Unfortunately, the Moon will overpower most of the nearby stars, so you might want binoculars to help you see them. The sea-goat’s leading light is known as Delta Capricorni or Deneb Algedi – the tail of the goat. It’s about 39 light-years away. It’s actually two stars locked in orbit around each other. The main star is twice the size and mass of the Sun, and about eight times the Sun’s brightness. The other star is smaller and fainter than the Sun. Twice a day, Delta Cap fades a bit. That’s because its stars orbit each other once per day. And they’re aligned in such a way that they eclipse one other during each orbit. The system dims a bit more when the faint star passes in front of the bright one, and a bit less when it’s the other way around. The stars of Capricornus form a wide triangle. Delta Cap is at the left point of the triangle. It climbs into good view by about 1 a.m. less than a degree from the bright Moon. Script by Damond Benningfield

A perfect spiral galaxy would include a bright, round “bulge” of stars in the middle; glittering spiral arms wrapping around it; dark lanes of dust lacing through the arms; and bright star clusters sprinkled about like lights on a Christmas wreath. In other words, it would look just like Messier 81, one of the best examples of a “grand design” spiral galaxy. It’s about 12 million light-years away, and appears close to the bowl of the Big Dipper. It’s a bit smaller and less massive than our own home galaxy, the Milky Way. M81’s “bulge,” though, is much larger and brighter than the one in the center of the Milky Way. And the black hole in the galaxy’s heart is almost 20 times as massive as the Milky Way’s. The spiral arms are outlined by the galaxy’s youngest, brightest stars. Over the past 600 million years or so, a major bout of starbirth has brightened the arms. That outburst is the result of gravitational interactions between M81 and two companion galaxies. The encounters compress big clouds of gas and dust. The clouds break into clumps, which then collapse to form stars – stars that make Messier 81 one of the most beautiful galaxies of all. Under clear, dark skies, you can spot M81 with binoculars. Find the Big Dipper, which is high in the north at nightfall. M81 hangs below the bowl at that hour. It looks like an oval smudge of light that’s almost as wide as the Moon. Script by Damond Benningfield

The stars look like they’re stuck in position – like fairy lights thumbtacked to a giant black canvas overhead. And over the course of a human lifetime – or many lifetimes – that’s true – there’s no way to see any motion without the help of sensitive instruments. But that’s only because the stars are so far away. Every one of those little lights is moving – fast. They’re all orbiting the center of the Milky Way Galaxy, for example. And they’re moving either toward or away from Earth. So over millions of years, the configuration of the stars changes – constellations come and go. And the pattern of brightness changes as well – some stars fade, others grow brighter. An example is Eltanin, the brightest star of Draco, the dragon. In fact, its name means “the great serpent.” It represents one of the dragon’s glowing eyes. Today, Eltanin is a bit more than 150 light-years away. But it’s moving more or less toward us at more than 60,000 miles per hour. On the scale of the galaxy, that’s tiny – but it adds up. In about one and a half million years, it’ll be just 28 light-years away. If the star doesn’t change much over that period, it could be the brightest star in Earth’s night sky. And it could maintain that rating for hundreds of thousands of years. Look for Eltanin high in the northeast at nightfall. It’s to the upper left of Vega, one of the brighter stars in the night sky – for now. Script by Damond Benningfield

Two fairly bright lights are headed for an especially close meet-up: the planet Mars and the star Regulus, the heart of the lion. They’re a few degrees apart tonight, but they’ll draw even closer over the coming evenings. Right now, Mars and Regulus are almost the same brightness. One way to tell them apart is their color – Mars looks pale orange, while Regulus is white with a hint of blue. Binoculars accentuate the colors. Another way to tell them apart is to look for them to twinkle. Regulus does, but Mars doesn’t. That’s because Mars is a bigger target in our sky. Regulus is thousands of times the size of Mars. But it’s so far away that we see it as nothing more than a pinpoint. That tiny beam of light is bent and twisted as it passes through the atmosphere. That causes the star to “twinkle.” It twinkles more when the air is more unsettled. Mars, on the other hand, is close enough that it appears as a tiny disk, made up of many pinpoints. Each one twinkles, but they even out. So Mars appears to hold steady as it shines through even the most un-steady skies. Look for Mars and Regulus about a third of the way up the western sky at nightfall. Regulus perches to the left or upper left of Mars. They’ll pass closest to one another on Monday and Tuesday. After that, they’ll move apart. At the same time, Mars will fade. A couple of weeks from now, Regulus will clearly outshine the Red Planet. Script by Damond Benningfield

Anchorage, Alaska, isn’t quite the “land of the midnight Sun.” Tonight, there are about five hours between sunset and sunrise. But it is a land of midnight sunlight, because twilight never completely fades. Twilight is the transition between day and night. Earth’s atmosphere scatters sunlight from the dayside to the fringes of the nightside. But when, exactly, does twilight end? When is the sky really dark? As you might expect, astronomers have their own definition. Astronomical twilight begins or ends when the Sun is 18 degrees below the horizon – about twice the width of your fist held at arm’s length. That’s when the sky’s as dark as it’s going to get. Because of the Sun’s motion, astronomical twilight lasts a minimum of about an hour and 10 minutes. But because the Sun usually rises and sets at an angle, twilight can last a good deal longer. During much of June and July, when the days are longest, twilight for much of the northern hemisphere lasts all night. The Sun never drops far below the horizon, so even though it’s out of sight, its light never disappears. So the people of Anchorage need some good blackout curtains to get a dark night’s sleep. If you want a few hours of darkness, head south – someplace like Miami Beach. It gets a full seven hours between evening and morning twilight – hours that might be illuminated by the neon lights of South Beach, but not by the Sun. Script by Damond Benningfield

Cold hands, warm heart. Still waters run deep. Feed a cold, starve a fever. The list of pithy old sayings goes on and on. But here’s a new one for you: long Sun, short Moon. It means that the full Moon does the opposite of what the Sun does. So when the Sun is in the sky for a long time, the full Moon makes itself scarce. And that’s the case tonight. The full Moon of June has many names, including Flower, Strawberry, and Honey Moon. But it’s also known as the Short-Night Moon. That’s because it’s in view for less time than any other full Moon of the year. From the northern hemisphere, the Sun passes highest across the sky at this time of year, and remains in view longest. But because of the trail they follow, the Sun and the full Moon are like opposite ends of a seesaw. When one is up, the other is down. So right now, the full Moon passes low across the sky. And it rises around sunset and sets around sunrise. The difference is greater as you go farther north. From Miami, the Sun will be above the horizon for almost 14 hours today, with the Moon popping into view for less than 10 and a half hours. From Duluth, Minnesota, it’s almost 16 hours versus less than eight hours. And from Anchorage, the Sun graces the sky for 19 hours, with the Moon showing up for a stingy hour and a half – an especially short appearance for the Short-Night Moon. More about night and day tomorrow. Script by Damond Benningfield

On average, Antares is the fifteenth-brightest star in the night sky. It looks like an orange-red gem at the heart of the scorpion. Tonight, though, it looks a little feeble. It hasn’t gotten any fainter. Instead, it stands especially close to the almost-full Moon. It looks a little washed out in the powerful moonlight. Antares really is one of the more impressive stars in the galaxy. It’s probably 12 to 15 times the Sun’s mass, hundreds of times the Sun’s diameter, and tens of thousands of times its brightness. Ere long – at least on the astronomical timescale – Antares will get even more impressive – but only for a while. Sometime in the next million years or so, it’s expected to blast itself apart in a titanic explosion – a supernova. For a few months, it’ll shine brighter than the combined glow of billions of normal stars. As it fades, its demolished outer layers will form a nebula – a colorful cloud of gas and dust, energized by the blast and by the decay of radioactive elements. Over thousands of years, the nebula will expand and fade. That will leave only the star’s dead core – a tiny, super-dense corpse known as a neutron star – the almost-invisible remnant of the mighty heart of the scorpion. Antares stands close to the Moon at nightfall, and the Moon will move closer to it during the night – washing out this brilliant star. We’ll have more about the Moon tomorrow. Script by Damond Benningfield

The rings of Saturn are among the most beautiful features in the solar system. They’re wide enough to span the distance between Earth and the Moon. And they’re made of bits of ice and dust – like tiny “moonlets” orbiting the giant planet. The first person to propose that idea was Giovanni Cassini, who was born 400 years ago today. He worked in several fields, from astrology to engineering. But Cassini’s greatest love was astronomy. He became director of the Paris Observatory, and studied the Moon and planets – especially Saturn. He discovered four of its moons, plus a dark “gap” between its two most prominent rings. That gap was named the Cassini Division in his honor. Although it looks empty, the gap contains a smattering of dark particles. It probably was mostly cleared out by a small moon that orbits inside the gap. Its gravity pushes ring particles away. A study a few years ago said the gap could have started much smaller than it is today. Over the past few million years, the moon moved closer to Saturn, clearing a wider region. But now, it’s moving away from Saturn. So the Cassini Division could close up – in about 40 million years. Saturn is in the southeast at dawn, and looks like a bright star. We’re viewing the rings almost edge-on, so there’s not much to see even through large telescopes. But the view will improve over the coming months – revealing both the rings and the dark divide between them. Script by Damond Benningfield

The world has known many great astronomers, but only a few great astronomy dynasties. One of those celebrates an anniversary tomorrow – the birth of its patriarch 400 years ago. Giovanni Cassini was born in a small Italian village just across the border from Nice, France. Cassini the first, as he’s often called, was the first of four generations of Cassini astronomers. And he definitely was the most productive. He’s best known for his discovery of a gap in the rings of Saturn. It’s named the Cassini Division in his honor. Later, Cassini and his son Jacques theorized that the rings were made of “swarms of tiny satellites” moving at different speeds. They understood the truth many years before it was actually confirmed. More about Cassini and Saturn’s rings tomorrow. Cassini made many other contributions to astronomy. He was among the first to realize that light travels at a limited speed, although he didn’t believe the speed could be calculated. And he developed a law to explain why we always see the same side of the Moon: The Moon takes the same amount of time to orbit Earth as it does to complete one turn on its axis. For most of his career, Cassini was director of Paris Observatory. His descendants kept the job in the family for more than 120 years. Cassini the fourth ended the streak in 1793 – when he left the observatory to write the “stellar” history of this astronomical dynasty. Script by Laura Tuma

There was a lot of talk earlier this year about an asteroid with the highest odds of hitting Earth ever calculated. The chances of an impact by asteroid 2024 YR4 in December of 2032 peaked at about three percent. The asteroid is big enough to cause major damage if it hit. As astronomers tracked it a little longer, though, they realized that’s not going to happen – the asteroid will miss by at least 60,000 miles. Such close calls aren’t rare. Hundreds of asteroids pass within a few million miles of Earth every year. This weekend, in fact, asteroid 2014 LL26 will miss us by just two million miles. Its orbit overlaps Earth’s orbit, so it passes close fairly often. And it could hit our planet at some point in the future – though not anytime soon. Astronomers have discovered more than 27,000 potentially hazardous asteroids. And they discover more all the time. The one that caused the kerfuffle earlier this year, in fact, was just discovered in December. Several automated searches scan the sky every night. Those efforts yield thousands of asteroids every month. But it takes observations over a period of days or weeks to give us a good measurement of an asteroid’s orbit. Most of the new discoveries are in the asteroid belt, between the orbits of Mars and Jupiter. But some are close enough to keep an eye on – potential hazards to life on Earth. Script by Damond Benningfield

The Pilbara region of Western Australia is big, dry, and wide open. And it may contain the oldest cosmic “scar” on Earth: an impact crater gouged three and a half billion years ago. Scientists discovered evidence of the crater during a brief expedition in 2021. They found some rock formations called shatter cones. Some of the cones are as tall as a house. The only known way to make them is in giant collisions with space rocks. Follow-up work last year revealed many more of these formations. The cones were found in a rock layer that’s miles wide, but only a few dozen feet thick. The layer also contains tiny “beads” that formed when molten rock was blasted high into the sky. The flight through the air sculpted droplets of the molten rock into balls. Geologists found that the layer formed three and a half billion years ago, so that’s when the impact must have taken place – more than a billion years earlier than the previous record holder. The asteroid could have been miles wide, and blasted a crater more than 60 miles across. The effects of the collision would have been felt around the world. In fact, researchers say the impact could have helped shape the world. Major asteroid impacts could have traveled deep, churning things up far below the surface. That could have created the “seeds” that gave birth to the continents when Earth was young. We’ll talk about potential future impacts tomorrow. Script by Damond Benningfield

You can’t tell just by looking, but the universe undergoes constant change. Stars explode. Quasars flare up. Asteroids zip past Earth. And soon, astronomers will be able to generate super-high-definition movies of those changes almost every night of the year. That’s because a new telescope dedicated to “time-domain” astronomy is about ready to take its first looks at the heavens. The telescope is the centerpiece of the Vera Rubin Observatory. It’s named for an astronomer who provided strong evidence for the existence of dark matter. It’s atop an 8700-foot mountain in Chile. The telescope’s main mirror, which gathers and focuses starlight, is 8.4 meters across – almost 28 feet. It has a wide field of view, allowing it to photograph the entire southern sky every few nights. It’ll record its observations on the largest digital camera ever built – 3200 megapixels. Astronomers will use those observations to learn more about dark energy and dark matter, and to map the Milky Way Galaxy. And they’ll watch for things that change. They’ll discover asteroids and comets – both close to Earth and deep in the outer solar system. They’ll see novas, supernovas, and other brilliant flare-ups. And the observatory will send out immediate notices of each new outburst, allowing other astronomers to make detailed follow-up observations – learning much more about our constantly changing universe. Script by Damond Benningfield

Before astronauts could land and walk on the Moon, NASA had to be sure they could do three things: live in space for days at a time, catch and link up with other spacecraft, and work outside their ship. And it took stabs at two of those goals 60 years ago today, with a mission called Gemini 4.   [AUDIO: 3, 2, 1, Ignition … Liftoff …] Astronauts James McDivitt and Ed White were scheduled to spend four days in space. That was longer than the first seven American missions combined. And White would make the first American spacewalk. He’d float outside the cabin for a few minutes, using a small “gun” of compressed air to move around. And three and a half hours after launch, it was time to get started: CAPCOM: Gemini 4, Hawaii capcom. We just had word from Houston, we’re ready to have you get out   whenever you’re ready. Okay, my feet are out. … Okay, I’m out. The spacewalk went well – very well. [WHITE: I feel like a million dollars!] In fact, it went so well that White didn’t want it to end. HOUSTON: The flight director says get back in!      McDIVITT: This is Jim, you got any message for us?      CAPCOM: Gemini 4, get back in!      McDIVITT: Okay… Actually working during a spacewalk turned out to be a lot harder than White had made it look. It took several more missions to work out the kinks. But the success of Gemini 4 helped make it possible for astronauts to walk on the Moon just four years later. Script by Damond Benningfield

If you step outside at dawn on June third of 2033, you’ll see the planet Venus standing due east – the brilliant “morning star.” But if you don’t want to wait that long to experience that beautiful view, then take a look at dawn tomorrow – Venus will be standing in the same spot in the sky. Not only that, it stands at that spot in the sky every eight years. In fact, anytime you see Venus – whether as the morning star or evening star – you can find it at that same spot eight years later. That’s because there’s a near-“resonance” in the orbital cycles of Earth and Venus. Venus completes 13 orbits around the Sun for every eight orbits that Earth makes. The ratio isn’t exact, but it’s quite close. Thanks to that, Venus follows five repeating cycles across our sky. It’s like the planet is a toy train on a looping, winding track. It hits every point along the track with every cycle. If you plot that motion from the perspective of Earth, it traces out a flowing pattern like five rose petals. The clockwork precision makes it easy to predict the entire sequence of Venus’s appearances. We know that it always remains in view in the morning sky for about 263 days, disappears for about 50 days, then moves into the evening sky. So if you miss the view of Venus tomorrow, or the next day, or the day after that, just mark it in your calendar – and look for it in the same location eight years later. Script by Damond Benningfield

The Moon has regular dates with the stars. It returns to the same position relative to the stars every 27 days and eight hours. As an example, the Moon cozied up to Regulus, the bright heart of the lion, on May 5th, and it does so again this evening – 27 days, eight hours later. This encounter is especially close as seen from the United States – the Moon and Regulus will appear to almost touch each other. That time span is known as the lunar sidereal period – “sidereal” meaning “related to the stars.” The planets have their own sidereal periods. Mars, for example, returns to the same point relative to the stars every 22 and a half months. Tonight, Mars is well to the lower right of Regulus, and looks like an orange star. It’ll return to almost the same position in April of 2027. The match won’t be exact because our viewing angle to the Red Planet changes a bit from year to year. The sidereal period is different from the period relative to the Sun – a difference caused by Earth’s own orbital motion. For the Moon, that period lasts 29 and a half days – the length of a cycle of phases. And for Mars, the Sun-related period is almost 26 months. That’s how long it takes Mars to return to the same angle from the Sun – part of the precise but sometimes confusing motions in the night sky. More about the motions of the planets tomorrow. Script by Damond Benningfield

To the more poetic among us, summer is a time of soft breezes, warm nights, and fireflies: The livin’ is easy, the breeze makes us feel fine, the warm Sun shines kindly upon us. But there’s less poetry in the summers on Mars – especially in the northern hemisphere, where summer began on Thursday. It stays cold, and the only fireflies are occasional meteors blazing through the night. Like the seasons on Earth, those on Mars are caused by the planet’s tilt on its axis. Northern summer begins when the north pole dips most directly toward the Sun. But Mars’s orbit is much more lopsided than Earth’s, so there’s a much greater change in the planet’s distance from the Sun. Mars is farthest from the Sun during northern summer. So the summer stays fairly cool. Summers and winters tend to be quiet times in the planet’s thin atmosphere. Big dust storms fire up in spring and fall, sometimes covering the whole planet. But they settle down by the start of summer. Mars does see more “dust devils” during summer – whirlwinds that can tower miles high. Northern summer will last for 178 Mars days – not giving way until the start of autumn exactly six months from now. Mars is close to the upper left of the Moon at nightfall, and looks like a fairly bright orange star. The true star Regulus is farther along that line. More about this lineup tomorrow. Script by Damond Benningfield

The United States plans to send astronauts to the Moon later in this decade, aiming toward a permanent lunar base. But experience shows that plans come and go. In fact, if all the plans for lunar exploration had actually come about, we’d be skittering all across the Moon today. In 1958, for example, the Air Force developed Project LUMAN, a comprehensive plan for human spaceflight. It would culminate with a single astronaut landing on the Moon. Later, the service developed another plan – LUMEX. It called for three astronauts to travel to the Moon using a giant new booster and a streamlined spaceship. The Army developed its own plan, involving a space station and other steps. All of those plans died – in part because human spaceflight was turned over to a new civilian agency: NASA. And NASA had its own false steps. It studied using its two-man Gemini spacecraft for lunar missions before settling on Apollo. And even then, some of its plans were scuttled; the final three Apollo missions were scrapped, in 1970. President George W. Bush proposed lunar missions as part of the Constellation program. It was nixed by President Obama. But some of its hardware has been kept for Artemis – which plans to send astronauts to the Moon in the next few years. Look for the Moon in the west at nightfall. The twin stars of Gemini stand to its lower right, with Mars to its upper left – another planned destination for human explorers. Script by Damond Benningfield

Globular clusters are the oldest members of the galaxy. They’re tight balls of hundreds of thousands of stars, most of which were born when the universe was no more than a couple of billion years old. Their most-massive stars have long since died. And most of the stars that remain are cool and faint. So a globular tends to be fairly quiet and calm. But that doesn’t mean that things don’t change. Consider Messier 13. It’s in Hercules, which is high in the east at nightfall. Under dark skies, the cluster is just visible to the unaided eye, looking like a faint, fuzzy star. The cluster is about 25,000 light-years away, and it contains up to half a million stars. But the stars at the edge of the cluster aren’t held as tightly as those in the middle. So the gravity of the rest of the galaxy can pull some of them away. In fact, astronomers have identified a few dozen stars that appear to be escapees from M13. But the cluster also can grab stars from the space around it. One especially young star probably became a member of the cluster that way. And stars inside the cluster can change. Some of them merge, forming bright, blue stars that look much younger. And stars die. M13’s brightest member is dying right now. It’s about as massive as the Sun, but it’s puffed up to dozens of times the Sun’s diameter. Soon, it’ll blow away its outer layers, leaving only its tiny, dead core – one more change in an ancient family of stars. Script by Damond Benningfield

Three of the four big moons of Jupiter appear to have something in common: oceans of liquid water below their crusts. For Europa, the ocean is considered a slam dunk. The case is also strong for Ganymede. But the case for the third moon, Callisto, is the weakest. Callisto is about 3,000 miles in diameter – bigger than our own moon. And it’s more heavily cratered than any other large body in the solar system. That indicates that the surface of Callisto is pretty much dead. But things might be different far below the surface. In the 1990s, the Galileo spacecraft flew near Callisto eight times. Its measurements of the magnetic field around the moon hinted that a salty ocean was sloshing around inside. But those observations also could be produced by an electrically charged layer of Callisto’s thin atmosphere. In a recent study, though, scientists looked at all of Galileo’s observations, and used computer models to understand them. The work suggested that there is an ocean. It could be dozens of miles deep. But it’s buried beneath an icy crust that could be hundreds of miles thick. Two spacecraft that are en route to Jupiter will fly close to Callisto many times. Their observations should tell us for sure whether an ocean is sloshing below Callisto’s battered surface. Jupiter stands below our moon in the early evening twilight. It looks like a bright star. Script by Damond Benningfield

If stars had trophy cases, Vega’s would be packed. The leading light of Lyra was the first star other than the Sun to have its picture taken, the first to have its spectrum taken, and the first with a published measurement of its distance. Vega is impressive in many ways. It’s more than twice the size and mass of the Sun, and about 50 times the Sun’s brightness. It’s encircled by a wide belt of dust, produced by collisions between big chunks of ice and rock. In about 12,000 years, it’ll serve as the Pole Star. And in about 200,000 years it’ll become the brightest star in the night sky. Vega first had its picture taken in 1850. In 1872, an astronomer took a picture of its spectrum, spreading its light into its individual wavelengths. A spectrum reveals a star’s composition, motion, and much more. In 1838, Russian astronomer Friedrich von Struve published Vega’s parallax – the first publication for any star. He plotted its position when Earth was on opposite sides of the Sun. That revealed a tiny shift against the background of more-distant objects. That shift revealed a distance of about 26 light-years – just one light-year off the modern measurement. Vega is low in the east-northeast at nightfall and climbs high overhead during the night. It’s the fifth-brightest star in the night sky, so you can’t miss it – a beautiful star with a case full of trophies. Script by Damond Benningfield

One of the major beauties of the summer sky dangles in the northeast this evening like a piece of cosmic jewelry – the constellation Lyra. Its brightest star is Vega – the fifth-brightest star in the night sky. It sparkles like the diamond stud in an earring. The rest of Lyra hangs to its lower right like the rest of the earring. It forms a parallelogram – a slanted rectangle. Under fairly dark skies, it’s easy to see. Lyra represents a lyre – a small harp. In skylore, it was sometimes shown being held by a large bird – an eagle or vulture. In fact, the name “Vega” comes from an Arabic phrase that means “the falling eagle.” But mainly the lyre was associated with the story of Orpheus. His music was legendary. When he accompanied Jason and the Argonauts, his playing silenced the Sirens – evil creatures who lured sailors to their doom. Orpheus married Eurydice. But she was bitten by a snake and died. Orpheus begged Hades, the god of the underworld, to let Eurydice return to him. His music was so beautiful that Hades agreed. But there was one condition: Orpheus couldn’t look back until they were outside. But he couldn’t resist – he looked too soon, and Eurydice vanished into the underworld forever. Orpheus was heart-broken. He roamed aimlessly across the countryside, playing sad but beautiful music on his lyre – an instrument commemorated in the stars. We’ll have more about Vega tomorrow. Script by Damond Benningfield

The names of the stars are a cultural mash-up. The names come from Greek, Latin, Arabic, and other cultures. And some names combine words from different languages. Two examples are the stars Tania Borealis and Tania Australis. “Tania” comes from an Arabic phrase that means “the second.” Borealis and Australis come from Latin, and mean northern and southern. Combined, the stars represent the second leap of the gazelle – a bit of skylore from Arabia. Skywatchers there saw three close pairs of stars as the leaps of a gazelle. All three pairs are at the edge of the modern constellation Ursa Major, the great bear. The “Tanias” are above the stars that form the outer edge of the dipper’s bowl. Tania Borealis is a single star that’s a good bit bigger, brighter, and heavier than the Sun. It’s at the end of the prime phase of life, so it’s undergoing big changes in its core. That’s causing its outer layers to begin to puff up to giant proportions. Tania Australis is a binary – two stars bound together by their mutual gravitational pull. One of the stars is similar to the Sun. The other is more than six times the Sun’s mass, and it’s already reached the “giant” phase of life. It’s puffed up to about 75 times the Sun’s diameter, and it shines about a thousand times brighter. So Tania Australis looks a bit more impressive than its northern cousin – the brighter half of the second leap of the gazelle. Script by Damond Benningfield

A gazelle leaps past the feet of the great bear. In ancient skylore, in fact, it made three leaps – each marked by a pair of stars. The stars that mark the first jump are known as Alula Borealis and Alula Australis – the northern and southern first leaps. As night falls this evening, they’re high in the northwest. They’re far to the left of the Big Dipper, which has the most prominent stars of the great bear. The “alulas” are close together, so they resemble a pair of eyes. Alula Australis holds an important distinction in the history of astronomy. A telescope reveals it’s a “double” star – two stars that are close together. By measuring the motions of the stars, in the late 18th century, astronomer William Herschel showed that they’re bound to each other. That made the system the first confirmed binary – two stars that move through space together, tied by their mutual gravitational pull. A few decades later, it became the first binary to have its orbit accurately measured. The stars orbit each other once every 60 years, at an average distance of more than 20 times the distance from Earth to the Sun. In more modern times, astronomers found that both stars are binaries on their own – each has a small, faint companion in a tight orbit. So the first leap of the gazelle consists of at least four stars, leaping through the galaxy as a family. We’ll talk about the second leap tomorrow. Script by Damond Benningfield

About 44 lightning bolts flash through the skies of Earth every second. And for a while, it seemed there might be more flashes than that on Venus. Telescopes on Earth and spacecraft in orbit saw flashes of light in the planet’s clouds, or heard the sounds of lightning in the planet’s radio waves. One estimate said Venus could see several times more lightning bolts than Earth. But studies in recent years have suggested that lightning on Venus might be quite rare. Venus has a much thicker atmosphere than Earth does, topped by clouds of sulfuric acid. But there’s very little water vapor in the clouds. And water is a key ingredient for lightning on Earth and other worlds where it’s been confirmed. So that led to some skepticism about claims of lightning on Venus from early on. A Sun-watching spacecraft has swung close to Venus several times. It’s listened for radio waves like those produced by lightning on Earth. It found them. But they were headed in the wrong direction – toward the ground, not into space, as they are on Earth. And a Venus orbiter has spent hundreds of hours looking for lightning flashes, but hasn’t seen a thing. In fact, one recent study said some of the flashes seen from Earth might really be meteors burning up in Venus’s atmosphere. So lightning might be rare on our neighboring planet. Venus is the bright “morning star.” Tomorrow, it’s close to the crescent Moon. Script by Damond Benningfield

If you ever want to chat with someone on another planet, you better have a lot of patience. It takes a long time for a message from Earth to reach another world, and just as long for the reply to reach Earth. That’s because radio waves travel at the speed of light. And although light is pretty swift, its speed is limited – 670 million miles per hour. Given the scale of the solar system, that means there’s a long pause between halves of a conversation. Consider the planets that flank the Moon at dawn tomorrow: brilliant Venus to the lower left of the Moon, and fainter Saturn to the upper right. Right now, Venus is more than 58 million miles away. At that distance, it would take a radio signal from Earth about five and a quarter minutes to get there – a round-trip time of 10 and a half minutes. Saturn is about 930 million miles away. So the round-trip travel time is about two hours and 45 minutes. That’s a big concern for the folks who send probes to these and other planets. Despite what you might see in sci-fi movies and TV shows, there’s no way to have a real-time conversation. So spacecraft are programmed to do much of their work without direct help from Earth. And if they encounter a problem, they shut down most of their systems and place a call for help – then settle in for the long wait to hear from home. More about the Moon and Venus tomorrow. Script by Damond Benningfield

A 900-mile-wide, two-toned walnut orbits the planet Saturn. It’s Saturn’s third-largest moon, and definitely the most eye-catching. One hemisphere is as dark as coal, while the other is as bright as sea ice. And a mountain ridge wraps around the equator, making it look like a walnut. Iapetus was discovered in 1671. And right away, astronomers realized there was something odd about it. It was easy to see when it was on one side of Saturn, but invisible on the other. Today, we know why that’s the case: the planet’s leading hemisphere is 10 to 20 times brighter than the trailing hemisphere. The leading idea says that long ago, the darker side was pelted by dust and rocks blasted off some smaller moons. The darker material trapped the Sun’s heat, vaporizing ices. The vapor drifted to the other side, where it froze, making that side bright. And that process continues today – making Iapetus the “yin and yang” of moons. The ridge around the equator is about six miles high. It might have formed long ago when Iapetus rotated much faster than it does today. Or it might be the remains of a ring that collapsed onto the surface – making Iapetus look like a walnut. Saturn appears quite close to our own Moon at dawn tomorrow. It looks like a bright star to the lower left of the Moon. The much-brighter planet Venus is farther to the lower left. More about this morning lineup tomorrow. Script by Damond Benningfield

For the most part, black holes come in two varieties: small and jumbo. The small ones are the remnants of dead stars. They range from a few to about a hundred times the mass of the Sun. The jumbos inhabit the hearts of galaxies. They range from about a hundred thousand to several billion times the Sun’s mass. But there just isn’t much in the middle. In fact, astronomers have logged only a few dozen medium-sized black holes. And many of those are controversial – they’re hard to confirm. Most of them are in other galaxies, so it’s hard to see their influence on the stars and gas around them. One recent study may have added to the tally. Using observations designed to study dark energy, scientists said they discovered about 300 medium-sized black holes. About 70 of them inhabit the centers of small galaxies. Those black holes are gobbling gas and dust around them, making them brighter and easier to find. Theory says there should be many more black holes in size “medium.” They may be the remnants of the ultra-massive stars that populated the early universe. Or they may have formed when dense clumps of gas collapsed under their own gravity. Either way, it’s possible that such black holes were the “seeds” from which the jumbo black holes grew. For now, the search continues for these hard-to-find medium-sized black holes. Script by Damond Benningfield

The closest galaxy we can see other than our own Milky Way may be inside the Milky Way. Its outer precincts have been stripped away, leaving only its core – a tight ball of 10 million stars. And a rare type of black hole appears to lurk in its middle. Omega Centauri rolls low across the south during the night. The view is better from the southern half of the country. To the eye alone, it looks like a fuzzy star. Omega Centauri is classified as a globular cluster – a family of very old stars. It’s the biggest one in the galaxy. But it probably wasn’t born in the Milky Way. Instead, it began as a separate galaxy. But it was reeled in by the Milky Way’s gravity, which also pulled away most of its stars. Only the stars in the galaxy’s core stuck together. An intermediate-mass black hole appears to inhabit the center of the cluster. Such beasties are rare. Most black holes are either no more than about a hundred times the mass of the Sun, or a few hundred thousand times the Sun’s mass or more. A study a couple of decades ago reported a possible black hole in Omega Centauri weighing 40,000 times the Sun’s mass. Later work suggested that number was too high. The most recent estimate was compiled from 20 years of observations by Hubble Space Telescope. It puts the black hole at about 8,000 times the Sun’s mass – a rare black hole in the remnant of a dead galaxy. More about mid-sized black holes tomorrow. Script by Damond Benningfield

Mighty Hercules stands well up in the east and northeast as night falls. His most prominent feature is the Keystone, a lopsided square of stars that represents his body. But the constellation’s brightest star isn’t part of the Keystone. Instead, it represents the entire strongman: Its name, Kornephoros, comes from a Greek word that means “the club bearer” – Hercules himself. Like many of the stars in the galaxy, there’s more to Kornephoros than meets the eye: It consists of two stars, not one. One star is smaller and fainter than the Sun, so it’s not visible to the eye alone. The visible star, on the other hand, is about three times the mass of the Sun, almost 20 times the Sun’s diameter, and 150 times its brightness. So the star is an easy target even though it’s about 140 light-years away. The star is nearing the end of its life. It’s probably consumed the hydrogen fuel in its core, converting it to helium. That’s caused the core to get smaller and hotter. The extra radiation pushes on its outer layers, causing them to puff up to giant proportions. Today, Kornephoros is fusing the helium to make carbon and oxygen. Eventually, that process will end. The star will lose its outer layers, leaving only its dead core – and the “club bearer” will vanish from sight. For now, though, look for it due east at nightfall, halfway up the sky – the first modestly bright star to the right of the Keystone. Script by Damond Benningfield

A pretty semicircle of stars crowns the sky on spring and summer nights: Corona Borealis, the northern crown. It’s in the east as night falls now, and stands high overhead a few hours later. In a couple of months, it’ll be overhead at nightfall. Most of the semicircle isn’t very bright – you need pretty dark skies to see it. It stands out because of the tight pattern, with a fairly bright star at the center: Alphecca, “the bright one.” Alphecca is really a binary – two stars locked in a gravitational embrace. The heavier of them is about three times as massive as the Sun, thousands of degrees hotter, and dozens of times brighter. Its companion is a little smaller and fainter than the Sun. The stars are quite close together – an average of about half the distance between the Sun and its closest planet, Mercury. The stars orbit each other once every 17 and a half days. And they’re lined up in such a way that we see the fainter star pass in front of the brighter one – an eclipse. When that happens, Alphecca dims by a few percent. That’s not enough for most of us to notice with the eye alone, but it’s an easy catch for astronomical instruments. Instruments also see a disk of debris around the stars. It extends billions of miles into space. It consists mainly of small grains of dust – material left over from the formation of Alphecca itself. We’ll talk about a pair of stars in Hercules tomorrow. Script by Damond Benningfield

Hubble Space Telescope had many “parents” – people who conceived it, lobbied for it, designed it, and mapped out its science mission. But none was more important than Nancy Grace Roman. She served as NASA’s first chief astronomer, and later as director of one of its field centers. She pushed, prodded, and cajoled for the telescope for decades. And once it was approved, she helped get it running. Roman was born 100 years ago today, in Nashville. In sixth grade, she founded her school’s first astronomy club. A year later, she decided to become an astronomer. Despite discouragement from teachers, she stuck with it. She earned her Ph.D., from the University of Chicago, in 1949. Over the next few years she studied the stars, using telescopes at McDonald Observatory and elsewhere. Academia didn’t offer much opportunity for women at the time, so Roman went into government work. And soon after NASA was established, she was hired as chief astronomer. Among other things, she led the development of the first space telescopes – one series to watch the Sun, another to study the stars. Roman died on Christmas Day in 2018. But her legacy is far from over. NASA’s next big space telescope will hunt for planets in other star systems, probe the nature of dark energy, plot the evolution of the universe, and more. The telescope is scheduled for launch in two years: the Nancy Grace Roman Space Telescope. Script by Damond Benningfield

For the most part, the star cluster NGC 2281 has escaped the attention of astronomers. It hasn’t been studied in a lot of depth over the years. So many of its details haven’t really been locked down. So far, astronomers have cataloged more than 200 stars in the cluster. And they’ve ruled out many more stars that happen to line up in the same direction. That makes NGC 2281 a fairly puny cluster. The cluster’s distance is a bit uncertain as well. Measurements have been getting better in recent years, thanks in part to the Gaia space telescope. It’s obtained precise details on several of the stars in the cluster, including their distance. Those observations put NGC 2281 at more than 1700 light-years. And its age is still debated, too. Estimates in recent years have ranged from about 275 million to 630 million years. Various studies have used different techniques to plot the age. That includes the types of stars found in the cluster, the number of dead stars, and even how fast the Sun-like stars in the cluster spin; stars slow down as they age. NGC 2281 is in Auriga the charioteer, in the west-northwest as night falls. The “twins” of Gemini stand to its upper left, with the brilliant star Capella farther to its lower right. Under clear, dark skies, it’s visible to the unaided eye as a hazy patch of light – a star cluster that we’re still getting to know. Script by Damond Benningfield

The “halo” that surrounds the Milky Way Galaxy is dark but heavy. It’s much more massive than the galaxy’s bright disk, but we don’t see much there. So the halo must be filled with dark matter. It produces no detectable energy, but it reveals its presence through its gravitational pull on the matter we can see. The leading idea says dark matter consists of some type of exotic particle. But efforts to find such particles have come up empty. Astronomers have also looked to see if the dark matter might consist of MACHOs – massive compact halo objects. The list of candidates includes faint stars, dead stars, black holes, free-floating planets, and brown dwarfs. Such objects are extremely faint. But they can sometimes brighten – not directly, but by magnifying the light of stars behind them. The technique is known as gravitational lensing. When one massive body passes in front of another, it causes the background object to get much brighter. The flare-up can last from hours to months. How long it lasts, and how much the background star brightens, reveals details about the lensing object. And that reveals the type of object. Searches for gravitational lenses have found many planets, faint stars, and even the first “rogue” black hole – one that couldn’t be seen any other way. But there just aren’t enough MACHOs to account for more than a small fraction of the dark matter – leaving us in the dark about its nature. Script by Damond Benningfield

The human eye is amazing. It can focus on objects near and far, provide a three-dimensional look at the world, and see under both brilliant sunlight and the faint glow of the stars. It also sees all the colors of the rainbow – from red and orange to blue and violet. Yet there’s a lot more that the eye can’t see – wavelengths that are beyond its range. That means we’re missing much of what the universe is showing us. Consider Antares, the heart of the scorpion. The star is just a whisker away from the Moon as they climb into view this evening. Antares is one of the brightest pinpoints in the night sky. And it shines with a distinctly orange hue. But there’s a lot more to it than that. For one thing, Antares consists of two stars, not one. The one we see is a supergiant – many times the Sun’s mass, and hundreds of times its diameter. At visible wavelengths, it shines about 10,000 times brighter than the Sun, with a distinctly orange color. But the star is much cooler than the Sun. Such stars produce most of their energy in the infrared – wavelengths too long for the human eye. So when you add that in, Antares is about a hundred thousand times the Sun’s brightness. The other star of Antares is much hotter than the Sun. So most of its light is in the ultraviolet – wavelengths that are too short for the eye. So it is about three thousand times the Sun’s brightness – much more than the eye can see. Script by Damond Benningfield

Our planet’s north magnetic pole is on a journey across the top of the world. But it’s slowing down. Over the past five years, it’s put on the brakes – its position has changed much more slowly than over the previous couple of decades. Earth’s magnetic field acts like a giant bar magnet, with north and south poles. The poles aren’t tied to the geographic poles – they wander. The north magnetic pole was discovered in 1831. At the time, it was centered over northwestern Canada. It moved farther south, then made a big turn, toward Siberia. In all, it’s moved almost 700 miles since it was discovered. For a couple of decades, it was moving at more than 30 miles per year. More recently, though, it’s slowed to about 22 miles a year – the biggest slowdown ever recorded. Scientists are trying to understand why. The magnetic field is generated by motions of molten rock in Earth’s outer core. Those motions produce electric currents, which create the magnetic field. So the changing position and rate of motion are telling us something about what’s going on deep inside our planet. The change in the magnetic pole has important practical implications as well as scientific ones. GPS, aircraft, the military, and others use magnetic north for navigation. So maps of Earth’s magnetic field are updated every few years to show the change in the pole’s location – keeping everyone headed in the right direction. Script by Damond Benningfield

The full Moon achieves a sort of celestial balance tonight. It’s passing across Libra, the balance scales – a symbol of justice. But the proper names of the constellation’s brightest stars have nothing to do with balance, justice, or anything similar. Instead, the names mean “the claws” – of nearby Scorpius, the scorpion. Originally, the stars did belong to Scorpius. But thousands of years ago, they were severed from the scorpion and placed in a new constellation. As night falls, one of the claws stands to the upper left of the Moon. Called Zubenelgenubi, it represents the southern claw. It’s the second-brightest star of Libra, and it’s about 75 light-years away. Like many of the stars in the night sky, Zubenelgenubi is deceiving. To the eye alone, it looks like a single point of light. Scan it with binoculars, though, and you’ll see two stars. They appear to be moving through space together, so they might be orbiting each other. But they’re so far apart that it takes the light from each star a month to reach the other one. At that separation, they might not be held together by gravity – their close appearance might be just a coincidence. Each of the two stars is actually a binary in its own right. In both cases, the stars are so close together that even giant telescopes can’t see them as individual stars. But we see the “fingerprints” of two stars in the light from each half of the southern claw. Script by Damond Benningfield

As World War II wound to an end, President Franklin Roosevelt asked his top scientific advisor a question: How could the type of research that helped win the war be applied to peacetime? The advisor suggested a new agency to support basic research at colleges and universities. It took a few years to work out the details. But 75 years ago today, President Harry Truman signed the law establishing that agency: the National Science Foundation. Over the decades, its mission has expanded into many fields, from chemistry and physics to computers and materials science. The list also includes astronomy. NSF established the first national observatories in 1956 – optical telescopes in Arizona, and radio telescopes in West Virginia. Today, NSF-supported facilities span the globe. They include observatories that no one was even dreaming of when the agency started. They hunt for the ghostly particles known as neutrinos, and listen for gravitational waves from merging black holes and neutron stars. NSF also is a partner in the Vera Rubin Observatory, which is scheduled to take its first peek at the universe this summer. Its giant telescope will scan a wide slice of the sky every night. It will discover exploding stars, asteroids, and other objects. It will map the Milky Way Galaxy. And it’ll provide new information about dark energy and dark matter – basic research that will teach us much more about the universe. Script by Damond Benningfield

The star Spica, which is quite close to the Moon tonight, is quite different from the Sun. It consists of two stars, not one. Both stars are many times bigger and heavier than the Sun. And their surfaces are tens of thousands of degrees hotter, so the stars shine blue-white. On the other hand, the Sun and Spica are made of almost exactly the same ingredients: mainly hydrogen and helium, with only a smattering of heavier elements. That composition was figured out by an astronomer who was born 125 years ago tomorrow, in England. Cecilia Payne caught the astronomy bug when she saw a lecture by Arthur Eddington, one of the world’s leading astronomers. She started her education in England, then finished in the United States. She earned a Ph.D. in 1925. And her doctoral thesis shook up the field. Decades later, in fact, Otto Struve, the first director of McDonald Observatory, called it the most brilliant thesis ever written in the field. Astronomers already had the techniques for measuring what stars are made of. Their work led them to believe that stars contain the same mixture of elements as Earth. But Payne used a new way to analyze the readings, taking into account the charge of atoms. She concluded that stars were made mainly of hydrogen and helium – elements formed in the Big Bang. By a few years later, just about everyone accepted her analysis – completely changing our concept of the stars. Script by Damond Benningfield

The surface of the Sun is like a pot of boiling water. Millions of bubbles of hot gas churn across it, constantly rising and falling. But the bubbles are a little bigger than those on your stovetop. The bubbles are known as granules. They form as energy from deep inside the Sun works its way to the surface. That heats the gas in the Sun’s top layer, forming bubbles. As they reach the surface, their gas cools and drops back into the Sun. This non-stop activity creates an easy-to-see pattern of bright blobs – the hot gas – with dark lanes between them – the cooler gas. The size of the granules varies from about a hundred miles to more than a thousand – big enough to swallow Texas. And each granule lasts for no more than about 20 minutes. A recent study said the granulation changes a bit during the Sun’s 11-year cycle of magnetic activity. Just after the peak of the cycle, there are slightly more granules than average, but they’re a little smaller than average. Other stars are so far away that we can’t see the granulation on most of them. But several types of observations confirm that they, too, are boiling away. Astronomers have seen granulation on a few stars. The stars are much bigger than the Sun. And they’re late in life, so they’re undergoing big changes. The granules on those stars are tens of millions of miles across – dozens of times the diameter of the Sun – giant bubbles of hot gas on giant stars. Script by Damond Benningfield

The Gaia spacecraft took its final look at the stars in January. But its work is far from over. Its observations will be producing new discoveries for decades. Above Earth’s blurring atmosphere, the space telescope kept a sharp eye on the heavens for more than a decade. It studied about two billion objects – mostly stars. It measured their temperature, composition, and motion. And it plotted their positions with amazing precision. That’s allowed astronomers to produce the best 3-D maps of the Milky Way Galaxy to date – by far. Those maps have helped plot the origins of many stars – the remains of star clusters or even small galaxies pulled in by the Milky Way. And that’s revealed a lot more about the history of the entire galaxy. Gaia also looked at other galaxies, at asteroids and comets in the solar system, and many other objects. It even helped reveal a couple of planets in other star systems. The craft ran out of the gas it used to keep its telescope on target, bringing its mission to an end. But it takes a lot of time to process Gaia’s observations and release them for study. There have been three big batches so far, which have yielded more than 13,000 scientific papers. The next big release is scheduled for late next year. And the final release – everything Gaia saw and reported – won’t be ready until late 2030 at the earliest – a treasure trove that astronomers will be poking through for decades. Script by Damond Benningfield